1. Filed of the Invention
The invention relates to a measuring arrangement for determining at least one measured variable with at least one sensor device and at least one signal output for outputting at least one output signal, wherein the output signal transmits information about the measured variable and/or a state of the measuring arrangement using at least one predetermined value of the current. For example, the measured variable is the flow, the fill level, the pH or the temperature of a medium. Further, the invention relates to a method for producing an output signal, wherein at least one piece of information about a measured variable and/or state of a measuring arrangement is transmitted at least by a current value of the output signal. The value of the current is, in particular, for example, half the peak-to-peak value.
2. Description of Related Art
In modern process automation, different measuring devices—an alternative term is field device—are used for determining or monitoring measured variables in order to monitor or control processes. For hazardous or safety-critical applications, the measuring devices—or as a general term: measuring arrangements, which may also include an interconnection of separate components for measurement—must meet a heightened level of security. Conversely, the measuring devices must provide a measure of reliability, which is commonly described using the assurance level or the Safety Integrity Level (SIL). In this case, the higher the level, the lower the risk of malfunction. For detection of an error state, corresponding measuring devices are equipped with a functionality that serves to monitor individual components or the entire measuring device.
If an error is detected, then this is indicated, on the one hand—particularly using a corresponding error signal—and on the other hand, the measuring devices change—particularly for critical errors—mostly into a safe state.
A measuring arrangement generally has at least one sensor device, which is used for determining the respective measured variable or even a number of measurement variables, and at least one signal output for outputting an output signal. The sensor device produces a measurement signal on the basis of a measurement, which is output, for example, directly via the signal output, or which is processed or at least partially processed in the measuring arrangement. The processing may mean, for example, that the actual value of a process variable that is of interest is calculated with stored calibration data from the measuring signal. For example, if the measurement signal is the transit time of a microwave signal of a fill level meter using the radar principle, information can be obtained using reference data—in particular, via the distance between an antenna used for the measurement and the tank, which contains the medium to be measured—about the fill level of the medium reflecting microwave signals.
The measurement signal can be output or, for example, displayed as an analog or digital output signal. In particular, the use of so-called 4 . . . 20 mA signals has spread in process automation. Here, information is transmitted using the current of the output signal, which, in the normal case, lies between 4 mA and 20 mA. In such signals, an error is indicated in that the current lies outside of the 4 to 20 mA range. Thus, in a variation, in particular, the current of the output/error signal is less than 4 mA.
In addition to the detection of an error, it is important that the measuring arrangement reliably indicates such an error, if one takes place.